Snow Melting GFEP

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joeyde44

Member
Location
Chadds Ford, PA
I was called by a manufactures rep to trouble shoot a large existing snow melt installation. This is a large residential apartment/condo project with a multi story underground parking garage. There are (4) exterior concrete stairways with imbedded snow melt mats. There are (4) 200 Amp, 240 Volt panel boards for the branch circuit breakers and each panel is energized by the manufactures control devices using magnetic contactors.

After I replaced some failed sensors and reprogramed the controllers I did a manual test to check things out. I was not able to energize the panel boards because the feeders were protected by CH type GFR ground fault protectors and they were tripping the shunt trip feeder breakers. I disabled the ground fault detectors on the feeders in order to energize the panel boards and found that every single mat was working properly with no measurable ground fault current.

I need to give this customer a clean and safe working system. What next?
 
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GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The combination of GFP tripping and "no measurable ground current" is hard to understand.
Remember that the GFP is sensing a difference in current between phase line and neutral return, not current measured in some ground, so you need to qualify your statement about no measurable ground current to tell us more about how you measured that.

Did you do a megger test on the disconnected heating elements?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
joeyde44 said:
Yes. What is a safe level to adjust it to?
Click to expand...
Ask the manufacturer's rep.

426.28 requires the GFP, but sets no level. Genral UL info says...

"EGFPD (Equipment Ground-Fault
Protective Device) ? Intended for
applications such as fixed electric deicing
and snow melting equipment, as well as
fixed electric heating equipment for
pipelines and vessels, in accordance with
Articles 426 and 427 in the NEC. This
device operates to disconnect the electric
circuit from the source of supply when the
ground-fault current exceeds the ground-
fault pick-up level marked on the device,
typically 6 mA to 50 mA. Additional
information for Ground-Fault Protective
Devices may be located under the UL
Product Category FTTE."
 
Sierrasparky

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Generally snow melt companies wan't to see a 30ma trip setting max.
I would megger all circuits.
Personally I think that system is of a poor design. You can have acceptable limits on each circuit. Then you add all these acceptable levels and have a trip.
 
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joeyde44

Member
Location
Chadds Ford, PA
Hey Everyone, Thanks for the comments. I had a long talk with a PE friend of mine, who confirmed that this system is a poor design and cannot work within the acceptable tripping limits of 30ma. Even if there were several mat circuits in the panel that were at acceptable leakage current levels, the cumulative effect will trip the feeder every time. He is recommending to install individual GFEP breakers for each mat circuit in the panels and forget about the feeder protection which would no longer be required.
 
PetrosA

PetrosA

Senior Member
Location
Lancaster, Pennsylvania
Sounds like your PE friend is right. My question before I saw your last post was going to be how long are the feeders out to the mats as this would also affect what the trip is going to be (just like a long UF will trip a GFI breaker with no actual ground fault). I'd do an analysis of the system including feeder lengths out to the mats just to make sure it will all work together later on.

According to the spec sheet, this probe from AEMC should be able to give you accurate readings in the 0-300mA range with a good DMM:

http://www.aemc.com/products/html/moreinfo.asp?id=20302&dbname=products

It might be worth the investment.
 
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